METHOD AND MAGNETIC RESONANCE APPARATUS FOR RECORDING MAGNETIC RESONANCE DATA USING A bSSFP SEQUENCE

ABSTRACT

In a method and magnetic resonance apparatus for recording magnetic resonance data using a bSSFP sequence, a k-space line to be scanned in k-space is divided into at least two line sections, with at least two of the at least two line sections being scanned separately in different repetitions of the sequence.

BACKGROUND OF THE INVENTION Field of the Invention

The invention concerns a method for recording magnetic resonance datausing a bSSFP sequence with a magnetic resonance apparatus, as well as amagnetic resonance apparatus, and a non-transitory data storage mediumencoded with programming instructions, for implementing such a method.

Description of the Prior Art

bSSFP sequences (balanced steady-state free precession sequences), alsoknown as “True FISP”, “FIESTA” and “T2-FFE” for instance, are widelyknown in magnetic resonance imaging. A bSSFP sequence is a coherentbalanced sequence, in which balanced gradients are used, namely for allthree axes. Here the term “balanced” means that the gradient-induceddephasing is canceled during a complete repetition of the sequence, inother words a complete TR interval. In unbalanced FFSP sequences, thegradients are unbalanced, which means that separate FID (free inductiondecay) and echo components of the free precession signal can be recordedin balance. With a bSSFP sequence, however, the balanced gradientsrefocus both components at the exact center of the repetition timeinterval (TR interval) as a single echo. Basic information relating tothe bSSFP sequence can be found in an article by Klaus Scheffler andStefan Lehnhardt for instance, “Principles and applications of balancedSSFP techniques”, Eur Radiol (2003) 13:2409-2418. bSSFP sequences arewidely used in magnetic resonance imaging if short total recording times(“fast imaging”) are involved. Fields of application include cardiacimaging and body imaging. The bSSFP sequence benefits from a highread-out efficiency due to the short echo times (TE) and repetitiontimes (TR), which are on the order of milliseconds. A further advantageof bSSFP is the high signal-to-noise ratio that is achieved therewith.

One problem that arises with bSSFP imaging, however, is that therecording technique responds very sensitively to field inhomogeneities,which may result in image faults known as banding artifacts. If thesebanding artifacts occur in the region of interest, this can make acorrect diagnosis more difficult. Approaches to reduce the bandingartifacts have been proposed, but many of these approaches aredisadvantageous because they significantly increase the total recordingtime (scan time).

US 2014/0292325 A1 describes a magnetic resonance imaging method forquantifying T1 and/or T2 repetition times in a sample, in which, interalia, the recording time is to be kept short. The use of an unbalancedbSSFP sequence with gradients known as crusher gradients is proposed, inorder to dephase the residual transversal magnetization. In such casesthree signals are recorded, namely an SSFP FID signal of the firstorder, an SSFP FID signal of the lowest order and an SSFP echo signal ofthe lowest order, from which T1 and T2 can be determined. Extremelyshort repetition times TR, shorter than 20 ms for instance, can be usedhere.

SUMMARY OF THE INVENTION

An object of the invention is to specify recording magnetic resonancedata with a bSSFP sequence by making the recording less influenced byinhomogeneities and is thus of higher quality.

This object is achieved by the method in accordance with the inventionfor recording magnetic resonance (MR) data by operating an MR scannerwith a control computer in order to execute a number of repetitions of abSSFP sequence, and wherein the control computer enters the acquired MRdata into a memory organized as k-space, with a k-space line to bescanned (filled with acquired MR data) in k-space being divided into atleast two line sections, and at least two of the at least two linesections are scanned separately in different repetitions of the bSSFPsequence.

The data entered into k-space in this manner are then made availablefrom the control computer as a data file.

The idea underlying the invention is that, by segmenting the scanningprocesses, the repetition time TR is reduced even further than wouldotherwise be possible with bSSFP sequences. While conventional imagingwith bSSFP sequences scans a complete line or row in the k-space to beread out, the associated gradient pulse sequences have a specific lengthin order to apply the gradient moment, so an analog-digital convertermust additionally be opened for a specific time for reading out the MRdata in order to be able to enter all magnetic resonance signals of thek-space line into that k-space line. With Cartesian scanning of k-space,in order to be able to scan a complete k-space line, a gradient pulsemust first be used in order to reach the start of the k-space row to bescanned in the read-out direction, which typically extends symmetricallyabout the k-space center in the read-out direction. A further gradientpulse with an inverted sign, known as the read-out gradient, is thenactivated, and while this is present the read-out of the magneticresonance signals typically also takes place via the analog-digitalconverter. The read-out gradient pulse is maintained and magneticresonance signals are read out until the end of the particular k-spaceline to be read out is reached, which typically faces the start withrespect to the k-space center. A further gradient pulse with an invertedsign is then required in order again to revert back to the k-spacecenter in the read-out direction, and thus to avoid dephasings. Sincegradients are contained in terms of the amplitude and the slew rate, aspecific period of time is essential for the read-out process, whichdepends overall on the resolution and the system properties. The totalof all gradient durations defines the repetition time TR of theacquisition with the bSSFP sequence.

The invention now proposes to perform a read-out segmentation, whereineach k-space line to be read out is divided into a number of portions,the line sections, wherein all parts are to be read out in differentrepetitions. In this way a shorter read-out module can be achieved inthe repetition since fewer gradient moments, therefore movements in thek-space in the read-out direction, are required and the duration of thegradient pulse sequence and of the read-out window is thus significantlyreduced. The various line sections are recorded in separate repetitions,in other words separate echoes, of the bSSFP sequence and are thenmerged accordingly during the image reconstruction.

Because the repetition time TR for the segmented read-out process can beselected to be considerably shorter, fewer banding artifacts can occurdue to inhomogeneities and the image quality is increased. Theprobability of acquisition of unusable magnetic resonance data issignificantly reduced.

It should be noted that the recording technique in accordance with theinvention can also be used particularly advantageously for less powerfulmagnetic resonance scanners with slow or less powerful gradient systemsand/or restricted shim systems, since these entail a particularly highrisk of banding artifacts, which can be reduced by the inventivedivision of the read-out process for k-space lines.

Expediently in such cases, Cartesian scanning of k-space can take placeand/or the k-space line can traverse the entire k-space to be scanned.Cartesian scanning of k-space is used more frequently for bSSFPsequences, wherein k-space lines, known as k-space rows, which followone another in the phase encoding directions for instance, and extend inthe read-out direction, can be scanned, and also define k-space to bescanned in terms of their length. Nonetheless the method can also beapplied, for instance, with at least partial radial scanning of k-space.

It is particularly advantageous if a k-space line to be scanned isdivided into an odd number of line sections. While in principle thek-space line can be divided into any number of line sections, oddnumbers are preferable because the k-space center is then scanned in thecenter of the read-out process assigned to an echo, and is thusuninfluenced by edge effects. A k-space line may be divided into three,five, seven or more line sections for example, depending on how much ofa time gain is desirable during the repetition time.

With the bSSFP sequence, in order to avoid dephasings, the read-out istypically carried out such that the k-space center is traversedrepeatedly in the read-out direction. Therefore, in an embodiment of thepresent invention, the read-out gradient pulses used to read out theline sections each end at the k-space center in the read-out direction.Modifications to a line section that is asymmetric in the read-outdirection with respect to the k-space center are ideally performed hereso that an excessively large number of switchover processes orindividual gradient pulses of the read-out gradient is avoided. Inanother embodiment, for a line section that ends before reaching thek-space center in the read-out direction, the duration of the gradientpulse at which the read-out of the magnetic resonance signals takesplace is extended until the k-space center is reached and/or, for a linesection of the gradient pulse that starts after the k-space center inthe read-out direction, the read-out of the magnetic resonance signalsis started at the k-space center. Overall, the read-out modules can varydepending on the position of the line sections, wherein a gradient pulsewith an inverse sign, which may result in a further shortening of theduration of the read-out module, can be omitted for line sections whichend before the k-space center or start after the k-space center.

It should be noted again that the k-space line is preferably dividedinto line sections of equal length and/or at least symmetrically inorder to enable possible repetition times to be as comparable aspossible for all repetitions.

In an embodiment of the invention not all line sections, into which ak-space line was divided, are measured, but rather at least one side ofthe k-space center is completely scanned in the read-out direction.Missing portions of the k-space line can be obtained using the Hermitiansymmetry of the magnetic resonance data that is ideally present. Whilein the case of a k-space line that extends symmetrically around thek-space center, theoretically only half of this k-space line thus mustactually be completely scanned. In practice more k-space data areacquired than the amount theoretically required, in order to be able tocompensate for imperfections, for instance phase errors. Anundersampling along a k-space line in the read-out direction isfrequently also referred to as “partial Fourier” or asymmetric echo.Undersampling in the read-out direction permits the total recording timefor the magnetic resonance data to be reduced further.

In addition to the method, the present invention also encompasses amagnetic resonance apparatus having a control computer designed toimplement the inventive method. All explanations relating to theinventive method apply to the inventive magnetic resonance apparatus,with which the advantages already indicated can consequently also beobtained.

The present invention also encompasses a non-transitory,computer-readable data storage medium encoded with programminginstructions that, when the storage medium is loaded into a controlcomputer of a magnetic resonance apparatus, cause the control computerto operate the magnetic resonance apparatus in accordance with one ormore of the above-described embodiments of the method according to theinvention.

The data carrier (storage medium) can be a CD ROM, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the scanning of a k-space line according to the priorart.

FIG. 2 is a flowchart of an exemplary embodiment of the inventivemethod.

FIG. 3 illustrates a read-out module for a first line section inaccordance with the invention.

FIG. 4 illustrates a read-out module for a second line section inaccordance with the invention.

FIG. 5 schematically illustrates an inventive magnetic resonanceapparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the scanning of k-space lines 1 of k-space 2 to bescanned in a bSSFP sequence according to the prior art. Three gradientpulses 3, 4 and 5 of the read-out gradient, which are used in theread-out module, are shown. The gradient pulse 3 is used in order toreach the start point 8 of the current k-space line 1 to be read out inaccordance with the arrow 6 in the read-out direction 7. Throughout theduration of the gradient pulse 4, k-space 2 is traversed in the read-outdirection 7 according to the arrow 9 and magnetic resonance signals areread out using an analog-digital converter, until the end point 10 ofthe current k-space line 1 to be scanned is reached. The gradient pulse5 is then used in order, according to the arrow 11 toward the end of theread-out module, to return once again to the k-space center 12 in theread-out direction.

It is precisely in magnetic resonance scanners with less powerfulgradient systems, in which therefore the amplitude and the slew rate ofthe gradient pulses 3, 4, 5 are restricted, that the duration of thisread-out module significantly contributes to the repetition time. Withlonger repetition times a larger number of banding artifacts may occur.

In contrast, in the inventive method shown below, the k-space lines 1are completely scanned, here in a Cartesian manner, not in a singleread-out process (in other words a single repetition), but instead bydividing them into line sections, which can be read out in separaterepetitions, in order thus to shorten the duration of the read-outmodule and therefore the repetition time and to reduce bandingartifacts.

A flowchart of an exemplary embodiment of the inventive method is shownas an example in FIG. 2.

Here in a step S1 a k-space line 1 to be scanned is divided into an oddnumber of line sections of equal size. Three line sections are used hereto simplify the representation. In order to be able to further reducethe repetition time TR, higher odd numbers can be used in this exemplaryembodiment.

In step S2, a first line section of the k-space line to be read out isthen read out in a single repetition, in other words a single echo,whereupon in step S3 a check is carried out to determine whether furtherline sections have to be read out, which can then be scanned accordinglyin further repetitions, step S2. This is shown in more detail with theuse of FIGS. 3 and 4 together with the adjustment to the gradient pulsesto be performed in the read-out modules.

According to the representation in FIG. 3, a k-space line 1, which is tobe scanned, was divided into three line sections 13, 14 and 15 of equalsize. Here the middle line section 14 contains the k-space center 12centrally in the read-out direction 7, which is advantageous. With thegradient pulse sequence in FIG. 3, the first line section 13 should nowbe scanned. To this end, the gradient pulse 3 in FIG. 1 is first usedhere unchanged, in order, in turn, to return from the k-space center 12in the read-out direction 7 to the start point 8 of the k-space line 1and also of the line section 13. A gradient pulse 16 with an inversesign, which permits the scanning in the read-out direction, follows thegradient pulse 3. However, the read-out time frame 17 does not cover thetotal duration of the gradient pulse 16, but instead only lasts untilthe end point 18 of the line section 13 is reached. The gradient pulse16 is then maintained, in order in this repetition to thus reach thek-space center 12 in the read-out direction 7.

In a corresponding representation, FIG. 4 shows the read-out module forthe central line section 14, the start point of which corresponds to theend point 18 of the line section 13. A gradient pulse 19 which isshorter compared with the gradient pulse 3 is therefore used first inorder to reach the start point 18 in the read-out direction 7. Thenthroughout the duration of the gradient pulse 20 with the inverse signthe read-out process takes place along the line section 14, comprisingin its center also the k-space center 12 in the read-out direction 7.After the end point 22 of the line section 14 has been reached, thek-space center 12 in the read-out direction is returned to by way of afurther gradient pulse 21.

If the line section 15 is also to be scanned, the gradient pulses 16 and3 in FIG. 3 would finally be used in reverse order, wherein the read-outtime frame naturally only starts when the start point of the linesection 15, which corresponds to the end point 22 of the line section14, is reached. However, to reduce the total recording time it isexpedient to perform an undersampling along the k-space line 1 and toentirely omit the scanning of the line section 15, since due to theHermitian symmetry in the read-out direction 7 in k-space, the data thatwere not obtained by operation of the scanner nevertheless can beobtained from the scanned magnetic resonance data.

In a step S4 (cf. FIG. 2), the magnetic resonance data of the individualline sections 13, 14 and 15 are then merged and the reconstruction of amagnetic resonance image data record takes place as usual.

FIG. 5 shows a block diagram of an inventive magnetic resonance device23, which, as is known in principle, has a scanner 24, into which apatient can be introduced via a cylindrical patient aperture 25. Thepatient bed used for this is not shown for simplicity. Typically a radiofrequency coil arrangement and a gradient coil arrangement of thescanner 24 are provided that surround the patient aperture 25, which arelikewise not shown in more detail for simplicity. Through the activationof the gradient coil arrangement, the read-out gradient pulses describedabove can be generated.

Operation of the magnetic resonance scanner 24 is controlled by acontrol computer 26, which is configured to carry out the inventivemethod. To this end, the control computer 26 can include a divisionprocessor for dividing k-space lines into line sections, and a sequencecontroller for instance, which then performs the corresponding scanningof the line sections in different repetitions by activating thecomponents of the magnetic resonance scanning 24.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the Applicant to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of the Applicant's contribution to theart.

1. A method for recording magnetic resonance (MR) data, comprising:using a control computer to operate an MR data acquisition scanner toacquire MR data by executing multiple repetitions of a bSSFP sequence;using said control computer to enter the acquired MR data into anelectronic memory organized as k-space comprising a plurality of k-spacelines in said electronic memory, by dividing a k-space line, into whichsaid acquired MR data are to be entered, into at least two linesections, and entering the acquired MR data respectively into at leasttwo of said at least two line sections separately in differentrepetitions of said bSSFP sequence; and from said control computer,making the data entered into k-space available in electronic form, as adata file.
 2. A method as claimed in claim 1 comprising entering theacquired MR data into k-space by Cartesian scanning of k-space.
 3. Amethod as claimed in claim 1 wherein said k-space line traverses anentirety of k-space into which said MR data are to be entered.
 4. Amethod as claimed in claim 1 comprising entering said acquired MR datainto k-space by Cartesian scanning of k-space, with said k-space linetraversing an entirety of k-space into which said acquired MR data areto be entered.
 5. A method as claimed in claim 1 comprising dividingsaid k-space line into an odd number of line sections.
 6. A method asclaimed in claim 1 wherein k-space exhibits Hermitian symmetry, andcomprising entering the acquired data into only some of said linesections of a k-space line, and thereby producing at least one linesection of the k-space line into which acquired MR data have not beenentered, and filling said at least one portion of the k-space line,after acquiring said MR data, using said Hermitian symmetry of k-space.7. A magnetic resonance (MR) apparatus comprising: an MR dataacquisition scanner; a control computer configured to operate said MRdata acquisition scanner to acquire MR data by executing multiplerepetitions of a bSSFP sequence; an electronic memory; said controlcomputer being configured to enter the acquired MR data into saidelectronic memory, organized as k-space comprising a plurality ofk-space lines in said electronic memory, by dividing a k-space line,into which said acquired MR data are to be entered, into at least twoline sections, and entering the acquired MR data respectively into atleast two of said at least two line sections separately in differentrepetitions of said bSSFP sequence; and said control computer beingconfigured to make the data entered into k-space available in electronicform, as a data file.
 8. A non-transitory, computer-readable datastorage medium encoded with programming instructions, said storagemedium being loaded into a control computer of a magnetic resonance (MR)apparatus comprising an MR data acquisition scanner and an electronicmemory, said programming instructions causing said control computer to:operate the MR data acquisition scanner to acquire MR data by executingmultiple repetitions of a bSSFP sequence; enter the acquired MR datainto the electronic memory, organized as k-space comprising a pluralityof k-space lines in said electronic memory, by dividing a k-space line,into which said acquired MR data are to be entered, into at least twoline sections, and entering the acquired MR data respectively into atleast two of said at least two line sections separately in differentrepetitions of said bSSFP sequence; and make the data entered intok-space available in electronic form, as a data file.